This invention relates to a silver halide photographic material for the production of lithographic printing plates by the silver salt diffusion transfer process, diffusion transfer processing solution and a method for making the printing plates.
Lithographic printing plates consists of greasy ink receptive oleophilic image portions and ink repellent oleophobic non-image portions, the latter being generally water receptive hydrophilic areas.
Therefore, the ordinary lithographic printing is carried out by feeding both water and ink to the surface of printing plates to allow the image portions to receive preferentially the ink and the non-image portions to receive preferentially water and then transferring the ink on the image portions onto a substrate such as paper.
Thus, in order to obtain prints of good quality, it is necessary that the difference between oleophilicity of the surface of image portions and hydrophilicity of the surface of non-image portions is sufficiently large so that when water and ink are applied the image portions can receive sufficient amount of ink while the non-image portions may completely repel the ink.
Defects of lithographic printing plates made by silver salt diffusion transfer process (DTR process) are as follows. The printing plates are poor in resistance against mechanical abrasion and ink receptivity of hydrophobic areas which carry ink images is gradually lost to result in decrease of printing endurance. Furthermore, the hydrophilic areas are gradually rendered hydrophobic and so the non-image areas, namely, background areas are apt to be stained with ink. Moreover, the hydrophobic image areas become unable to receive uniformly the greasy ink to cause unevenness of ink receptivity of the image areas.
There have already been printing plates in actual use which are produced by providing metallic silver patterns on the plates by DTR process which are rendered ink-receptive. See, for example, U.S. Pat. Nos. 3,220,837 and 3,721,559, Japanese Patent Examined Publications (KOKOKU) No. 16725/73 and No. 30562/73 and Japanese Patent Unexamined Publications (KOKAI) No. 4482/71 and No. 21603/78. However, these printing plates have still the defects as enumerated above.
Like other lithographic printing plates, lithographic printing plates obtained by applying the DTR process are also required to be increased in their printing endurance by selecting construction of plate materials, composition of processing solutions, printing conditions, etc., but the effect which the state of transfer-developed silver particles give on printing characteristics of printing plates is a big factor for increasing the printing endurance. In order that the silver particles transfer-developed by the DTR process may have higher printing endurance, conditions for formation of transfer silver particles, such as diffusion speed of silver complex, stability and reducing rate and size and shape of the formed silver particles are important factors although construction of lithographic printing plates has some influence thereon.
A great number of compounds are known as silver complex forming agents used for DTR process. For making lithographic printing plates by applying DTR process, thiosulfates and thiocyanates have been considered most preferred and actually used from the economical point of view and the point of printing endurance of said transfer-developed silver particles. However, even these silver complex forming agents cannot provide lithographic printing plates having satisfactory printing endurance.